Tiny Crystal Shapes Get Close Look From Mars Rover

Star-shaped and swallowtail-shaped tiny, dark bumps in fine-layered
bright bedrock of a Martian ridge are drawing close inspection by NASA's
Curiosity Mars rover.

This set of shapes looks familiar to geologists who have studied gypsum crystals
formed in drying lakes on Earth, but Curiosity's science team is considering
multiple possibilities for the origin of these features on "Vera Rubin
Ridge" on Mars.

One uncertainty the rover's inspection may resolve is the timing of when
the crystal-shaped features formed, relative to when layers of sediment accumulated
around them. Another is whether the original mineral that crystallized into
these shapes remains in them or was subsequently dissolved away and replaced by
something else. Answers may point to evidence of a drying lake or to
groundwater that flowed through the sediment after it became cemented into
rock.

The rover team also is investigating other clues on the same area to
learn more about the Red Planet's history. These include stick-shaped features the
size of rice grains, mineral veins with both bright and dark zones, color
variations in the bedrock, smoothly horizontal laminations that vary more than
tenfold in thickness of individual layers, and more than fourfold variation in
the iron content of local rock targets examined by the rover.

"There's just a treasure trove of interesting targets concentrated
in this one area," said Curiosity Project Scientist Ashwin Vasavada of
NASA's Jet Propulsion Laboratory, Pasadena, California. "Each is a clue,
and the more clues, the better. It's going to be fun figuring out what it all
means."

Vera Rubin Ridge stands out as an erosion-resistant band on the north
slope of lower Mount Sharp inside Gale Crater. It was a planned destination for
Curiosity even before the rover's 2012 landing on the crater floor near the
mountain. The rover began climbing the ridge about five months ago and has now reached
the uphill, southern edge. Some features here might be related to a transition
to the next destination area uphill, which is called the "Clay Unit"
because of clay minerals detected from orbit.

The team drove the rover to a site called "Jura" in mid-January
to examine an area where -- even in images from orbit -- the bedrock is
noticeably pale and gray, compared to the red, hematite-bearing bedrock forming
most of Vera Rubin Ridge.

"These tiny 'V' shapes really caught our attention, but they were
not at all the reason we went to that rock," said Curiosity science-team
member Abigail Fraeman of JPL. "We were looking at the color change from
one area to another. We were lucky to see the crystals. They're so tiny, you
don't see them until you're right on them."

The features are about the size of a sesame seed. Some are single elongated
crystals. Commonly, two or more coalesce into V-shaped "swallowtails"
or more complex "lark's foot" or star configurations. "These shapes
are characteristic of gypsum crystals," said Sanjeev Gupta, a Curiosity
science-team member at Imperial College, London, who has studied such crystals
in rocks of Scotland. Gypsum is a form of calcium sulfate. "These can form
when salts become concentrated in water, such as in an evaporating lake."

The finely laminated bedrock at Jura is thought to result from lakebed
sedimentation, as has been true in several lower, older geological layers Curiosity
has examined. However, an alternative to the crystals forming in an evaporating
lake is that they formed much later from salty fluids moving through the rock.
That is also a type of evidence Curiosity has documented in multiple geological
layers, where subsurface fluids deposited features such as mineral veins.

Some rock targets examined in the Jura area have two-toned mineral veins
that formed after the lake sediments had hardened into rock. Brighter portions
contain calcium sulfate; darker portions contain more iron. Some of the
features shaped like gypsum crystals appear darker than gypsum, are enriched in
iron, or are empty. These are clues that the original crystallizing material
may have been replaced or removed by later effects of underground water.

The small, stick-shaped features were first seen two days before
Curiosity reached Jura. All raw images from Mars rovers are quickly posted
online, and some showing the "sticks" drew news-media attention comparing
them to fossils. Among the alternative possibilities is that they are bits of
the dark vein material. Rover science team members have been more excited about
the swallowtails than the sticks.

"So far on this mission, most of the evidence we've seen about
ancient lakes in Gale Crater has been for relatively fresh, non-salty water,"
Vasavada said. "If we start seeing lakes becoming saltier with time, that
would help us understand how the environment changed in Gale Crater, and it's
consistent with an overall pattern that water on Mars became more scarce over
time."

Such a change could be like the difference between freshwater mountain
lakes, resupplied often with snowmelt that keeps salts diluted, and salty lakes
in deserts, where water evaporates faster than it is replaced.

If the crystals formed inside hardened rock much later, rather than in an
evaporating lake, they offer evidence about the chemistry of a wet underground
environment.

"In either scenario, these crystals are a new type of evidence that
builds the story of persistent water and a long-lived habitable environment on
Mars," Vasavada said.

Variations in iron content in the veins, smaller features and surrounding
bedrock might provide clues about conditions favorable for microbial life. Iron
oxides vary in their solubility in water, with more-oxidized types generally
less likely to be dissolved and transported. An environment with a range of
oxidation states can provide a battery-like energy gradient exploitable by some
types of microbes.

"In upper Vera Rubin Ridge, we see clues that there were fluids
carrying iron and, through some mechanism, the iron precipitated out,"
Fraeman said. "There was a change in fluid chemistry that could be
significant for habitability."